1. Field of the Invention
The present invention relates to a display device employing a ferroelectric liquid crystal.
2. Related Background Art
Among display devices employing ferroelectric liquid crystals, a device such as that disclosed in Japanese Patent Laid-Open No. 94023/1986 is known, in which a liquid crystal cell is comprised of a pair of glass substrates disposed in opposition to each other with a cell gap of about 1 to 3 microns therebetween, the substrates having electrodes on the mutually opposing surfaces thereof and being subjected to alignment treatment. A ferroelectric liquid crystal is charged in the cell gap.
Such a display device employing a ferroelectric liquid crystal has the following characteristics: because a ferroelectric liquid crystal is capable of spontaneous polarization, a switching action can be caused by utilizing a combination force resulting from an external electric field and spontaneous polarization; and because the long-axis direction of molecules of a ferroelectric liquid crystal is in one-to-one relationship with the direction of spontaneous polarization, a switching action can take place depending on the polarity of an external electric field.
In general, a chiral smectic liquid crystal (SmC*, SmH*) is used as the ferroelectric liquid crystal. When such a liquid crystal is in bulk form, molecules of the liquid crystal are oriented with their long axes being twisted. However, when the liquid crystal is charged in a cell having a cell gap of about 1 to 3 microns, as in the above-described cell, it is possible to relieve the liquid crystal of the long-axes twisted orientation (N. A. Clark et al., MCLC (1983), Vol, 194, pages 213 to 234).
In an actual structure of a ferroelectric liquid crystal cell, simple matrix substrates, such as those shown in FIG. 5A and 5B, are used. The cell comprises, as shown in a section in FIG. 5A, upper and lower (as viewed in FIG. 5A) glass substrates 21, and a liquid crystal 26 charged in the space between the substrates 21. Each of the substrates 21 has a group of indium tin oxide (ITO) stripe electrodes 22, a silicon dioxide (SiO.sub.2) insulating film 23, and a polyimide alignment film 24, these being formed on the substrate 21 in this order. The charged liquid crystal 26 is sealed by sealing members 25. Each group of the ITO stripe electrodes 22 is arranged in a pattern such as that shown in FIG. 5 (b). The group of the electrodes 22 on the upper substrate 21 and that on the lower substrate 21 extend in directions perpendicular to each other.
When, however, the above-described display device performs line-sequential scanning, information signals indicating the same information contents have the same waveform and, in addition, the same writing timing. As a result, the following problems arise:
(1) As the size of the display screen of the liquid crystal display device increases, and as the level of resolution of the device increases, the wiring resistance of the scanning signal lines (hereinafter referred to as "common lines" unless otherwise specified) and that of the information signal lines (hereinafter referred to as "segment lines" unless otherwise specified) increase, thereby increasing the delay in waveform transmission through the electrode lines; PA1 (2) If a ferroelectric liquid crystal (hereinafter abbreviated to "FLC" unless otherwise specified) is used, the thickness of the liquid crystal layer must be maintained within the range from 1 to 2 .mu.m in order to ensure good bistability. The liquid crystal layer must be thin (about 1 to 2 .mu.m thick) for other reasons also; for example, with a greater thickness, the driving voltage will have to be increased, and retardation may result in non-negligible coloring. As a result, the electrostatic capacity of the liquid crystal layer increases, and is approximately six times that of a twisted namatic liquid crystal in a conventional device; and PA1 (3) The on-resistance of the driving IC cannot easily be reduced to a level of about 1 k.OMEGA. or lower because of mass production requirements, etc.
These problems lead to a phenomenon in which a driving waveform applied to the liquid crystal layer is rounded. This rounding is disadvantageous in that it causes problems such as an in-cell distribution of the threshold of the FLC switching that result in a reduction in the switching margin. Another disadvantage is that the non-select pixels are greatly influenced.
The second disadvantage will be described with reference to FIGS. 2A to 2D. At a non-select pixel, the common line is supplied with the reference voltage, whereas the segment line is supplied with the information signal for the select pixels. When, as shown in FIG. 2A, the information signal for one frame first indicates that all of the pixels should be black b, and then indicates that a part of the pixels should be a white circle w, since a greater part of the information signal for one frame indicates "black" (as shown in FIG. 2B), differentiation-waveform ripple (shown in FIG. 2C) corresponding to the rises and the falls of the information signal occurs on the common line. The peak value of such ripple is determined by the information contents of the information signal. Substantially no ripple occurs on the common line when the information signal indicates, for example, a pattern with one-bit checkers because, in such cases, the voltages input to two adjacent segment lines have opposite (positive and negative) polarities, and have the same peak value.
However, when, as shown in FIG. 2A, it is desired that a greater part of the frame be "black", ripple occurs corresponding to the rises and the falls of the black information signal, as shown in FIG. 2C. In this case, if the information signal for producing the white circle w has, as shown in FIG. 2D, a waveform in an inverted phase, the information signal for the "white" pixels is emphasized by the ripple, conversely to the case of the information signal for the "black" pixels. Thus, a difference occurs between the magnitude of influence on the non-select pixels by the "black" signal and that by the "white" signal. In other words, the difference in magnitude of the information signal at a select pixel causes a difference in the level of fluctuation of particles at a non-select pixel. The difference in the fluctuation level results in flickering on the screen, thereby greatly deteriorating the quality of display. In brief, when a black portion b (in FIG. 2A) is being produced, the contents of the information signal are common to all of the pixels on the screen; however, when a white circle w (in FIG. 2 A) is being produced, the actual waveforms (resulting from the addition or subtraction of ripple) become different between segment lines supplied with a "white" signal and segment lines supplied with a "black" signal, and are thus distorted.